If there were ever any doubt that measles is a dangerous disease, astudy published in the journal Science yesterday blows that misconception away by finally explaining how large-scale measles vaccination caused such a plummet in childhood deaths – even from infections that seemed unrelated to measles.
Aside from the risk of death and disability that measles itself poses, the disease has another sneaky trick that scientists suspected but had trouble showing: it gives the immune system amnesia. By making the immune system forget how to fight past infections, measles throws the doors wide open for all those illnesses to visit again now that the body can’t immediately remember how to defeat them.
The measles vaccine then, in addition to preventing measles, actually prevents dozens of other infections that otherwise might occur even after a person recovers from measles. And that means widespread measles vaccination protects a community not only from measles but from other diseases as well.
Scientists had already known that measles infections suppressed the immune system for several weeks to several months, just as cancer and some other diseases do. While the immune system is beaten down, opportunistic infections such as pneumonia or diarrhea can wreak havoc on a person’s body even after they’ve recovered from the measles itself.
But there were hints that measles did more than suppress the immune system for a couple months, hints that became especially apparent after widespread vaccination. As lead author Michael Mina, an evolutionary biologist at Princeton University, and his colleagues explain in their paper, mass measles vaccination was wildly successful in reducing deaths – perhaps suspiciously successful since the drop in deaths that correlated with the vaccine’s introduction included those from non-measles infections. While advances in medical treatment and hygiene might appear to play some role in this drop, the correlation between non-measles infectious disease deaths and the vaccine’s introduction was too strong to ignore.
“They were trying to figure out just how and why measles vaccination had such long-lasting benefits and protection beyond simply preventing an active measles infection – how did it protect kids from other infectious diseases as well?” explained Tara C. Smith, an infectious disease researcher at Kent State University.
So they developed mathematical models to more closely examine deaths from non-measles infections before and after the introduction of the measles vaccine in four high-income countries: the U.S., England, Wales and Denmark. “They found that when measles infections were reduced, that led to a decrease in deaths from other infectious diseases as well – for up to three years,” Smith said. “They concluded that this was due to an effect of the measles virus itself, which they suggest causes the body to, in essence, ‘forget’ some of the immunity it has developed against other diseases.”
Other recent research had already tested the idea that this immune memory loss occurred after measles infections. Researchers created the same immunosuppressive conditions in macaque monkeys that a measles infection creates and found that the disease-fighting white blood cells basically forgot how to fight much of anything besides measles.
The researchers’ mathematical modeling was precise enough to calculate that the memory-wipe from measles infections lasts an average 27 months. Data showing infectious disease deaths occurring within a window of a few years after measles infections supported this finding as well.
“Studies like this are tough to do, but they purposely picked an outcome that was easily measurable – death – rather than something that would be more difficult to find in historical records, such as illnesses,” Smith said. “The three different countries and databases they chose all support the findings, even though measles vaccines were introduced in the UK and US in the 1960s but not until the 1980s for Denmark, so that rules out a simple worldwide trend that may have been occurring in the mid-century.”
Further, the researchers looked for similar patterns from pertussis and didn’t see the same effect.
“They have multiple controls that suggest this really is just an effect of the measles virus, and therefore prevented by the vaccine,” Smith said. “The data fit in with previous studies that have seen this effect (but they weren’t sure of the cause), and with others that looked specifically at immune responses following measles infections, so it looks fairly convincing.”
In a sense, then, you could think of the MMR as protecting against more than just measles, mumps and rubella. It protects against those three diseases as well as all the others that might have come along had a person not been protected from measles.
“Our results show that when measles was common, measles virus infections could have been implicated in as many as half of all childhood deaths from infectious disease,” the authors wrote. “The reduction of measles virus infections was the main factor in reducing overall childhood infectious disease mortality after the introduction of vaccination.” Further, because vaccination provided a population (herd) immunity against measles, that meant that, in effect, “measles vaccination might also be preserving herd protection against non-measles infections.”
Eliminating measles with the vaccine is a triple win: less measles, less death from other diseases in individuals and less infections overall within a highly vaccinated population.